1. Field Of The Invention
This invention relates to lateral bipolar junction transistors and their application in operational amplifier circuits.
2. Description of Related Art
Bipolar Junction Transistors (BJT) have been critical to the development of the field of electronics and their design are well understood. Lateral BJT's have primarily had applications such as current sources that do not require large .beta. (the ratio of the collector current to the base current) or to operate at high frequencies.
In metal oxide semiconductor (MOS) transistor circuits, a parasitic later BJT is present in the area of the drain diffusion, the source diffusion, and the area between the drain and the source of the bulk semiconductor into which the MOS transistor is being fabricated. These parasitic devices often undesirable properties that can detract from the operation of the MOS transistor.
Bipolar Junction Transistors have been critical to the design of operational amplifiers for many years. Since the voltage developed from the base to the emitter (V.sub.be) of multiple BJT's can be precisely matched in integrated circuits processing, the offset voltage (the voltage developed at the output of the operational amplifier when the input terminals have the same voltage) is very small. However, since there is always a base current (I.sub.b) present into the input terminals of the operational amplifier, the input impedance is relatively low.
To improve this impedance, MOS field effect transistors (FET) have replaced the input transistors of operational amplifiers. Since there is very little current flowing into the gate of input MOS FET's, the input impedance of the operational amplifiers employing MOS FET's is very high. The transconductance is not as high as that of a BJT, due to the fact that a BJT has an exponential variation of current versus input voltage whereas a MOS FET has only a quadratic variation of current versus gate biasing. However, the offset voltage is not as low as with the BJT input operational amplifiers, because of variations in the transistor parameters such as threshold voltage (V.sub.T), the gain factor (K), and the body factor (.gamma.). These parameters are effected by the variations in the semiconductor processing. This offset voltage is often compensated for by additional components either with the operational amplifier on an integrated circuit chip or with circuitry external to the integrated circuit chip. U.S. Pat. No. 5,268,650 (Shcnabel) teach a technique for the use of BJT's and FET's in amplifier circuits for high amplification with low offset voltage and low quantities of noise. U.S. Pat. No. 4,486,942 (Hirao) demonstrates a method for the fabrication of Bi-MOS devices which have bipolar and MOS elements fabricated on the same integrated circuit device.